1. Field of the Invention
This invention relates to a process for producing a thin-walled cylindrical body of an aromatic polyester, a specific thin-walled cylindrical body produced by the aforesaid process, and to the application of the aforesaid cylindrical body to a magnetic recording medium.
2. Description of the Prior Art
Production of a cylindrical thermoplastic film from a thermoplastic resin has been known as an inflation film-forming method.
Japanese Patent Publication No. 18,978/1963 discloses a process for producing a biaxially oriented thermoplastic film which comprises extruding a thermoplastic resin from an extrusion die, imparting non-tackinesss to the extruded cylindrical body, adjusting the speed of feeding the cylindrical body by a speed adjusting device having a movable surface which accurately grasps the cylindrical body without flattening it, adjusting the temperature of the cylindrical body, expanding the cylindrical body by a gas pressure, and stretching it at a speed higher than the feeding speed.
Japanese Patent Publication No. 15,928/1962 discloses a process for producing a biaxially oriented thermoplastic film, which comprises extruding a thermoplastic polymer in a tubular form, taking it up, cooling it, maintaining in the tubular sheet a pressure for expanding it, preheating it, thereafter heating the inside surface of the tubular sheet by radiated heat to expand the tubular sheet, taking up the sheet at a speed at least twice the first take-up speed, and cooling it while maintaining the expanded diameter.
Japanese Patent Publication No. 7,479/1968 discloses a process for producing a biaxially stretched tubular film, which comprises blowing a gas into an unstretched tubular film of a thermoplastic resin while maintaining it at a stretching temperature and passing it through a guide having a size which is 70 to 130% of the diameter of the unstretched film.
Japanese Patent Publication No. 15,917/1969 discloses a process for producing a jointless biaxially stretched linear polyester film, which comprises extruding a linear polyester having a low viscosity into a tube, bringing the non-oriented tube into intimate contact with a cooled inside diameter determining mandrel to cool and solidify it, heating it to a stretching temperature, taking it up at a higher speed than the extrusion speed while expanding it by a pressurized gas, and then cooling it.
Japanese Patent Publication No. 19,720/1969 discloses a heat treatment process in which an endless belt is positioned within a cylindrical film in the film advancing direction, and the film is heat-treated in a hot air atmosphere, thereby restricting the shrinking of the cylindrical film in a direction at right angles to the film advancing direction.
Japanese Patent Publication No. 32,080/1971 discloses a process for a heat-shrinkable thermoplastic synthetic resin tube, which comprises passing a thermoplastic synthetic resin tube through an expansion controlling device composed of at least three plate-like members disposed in a liquid heat medium, and expanding and stretching the tube by passing a pressure fluid into its inside.
Japanese Patent Publication No. 25,387/1976 discloses a process for producing a tubular biaxially stretched film, which comprises first stretching an unstretched tubular film of a thermoplastic resin only in the longitudinal direction, cooling the film, and then biaxially stretching it simultaneously in the longitudinal and transaverse directions.
In any of these prior processes, the final tubular film which has been expanded and stretched and as the case may, further heat-treated is wound up in the folded flat state and offered as a product. Hence, all conventional cylindrical films obtained by the inflation film-forming method have folded marks. Cylindrical films having folded marks can be used as such in applications in which the presence of folded marks is not objectionable, but cannot be used as such in applications in which the presence of folded marks is decisively serious. For example, when a heat-shrinkable cylindrical film is used for packaging a material by utilizing its heat shrinkability, the presence of folded marks in the cylindrical film does not bear so serious a significance.
On the other hand, when a magnetic layer is provided on the inside or outside surface of the cylindrical film and the film in the cylindrical form is used as a magnetic recording medium, the cylindrical magnetic recording medium having information magnetically recorded thereon and a playback head must be rotated relative to each other with respect to the axis of the recording medium in the cylindrical direction. Hence, a cylindrical film having folded marks, however smooth a surface it may have, cannot be used as a carrier film for magnetic recording media wherein raised and depressed portions on the film surface which have an average height on the order of one hundredth of a micrometer are objectionable.
The folded marks of the cylindrical film are generally very strong. Investigations of the present inventors have shown that even when such a cylindrical film is mechanically heat-treated to remove the folded marks apparently, a magnetic recording medium produced by using it cannot permit smooth information-residual magnetism conversion by the head, and interruption of the recorded information occurs.